Building Web Apps With ASP.NET Core And DotVVM

There is one category of web applications where the MVVM pattern can be especially useful: line of business applications. Yes, these crazy large web sites with hundreds of grids, large and complicated forms with hundreds of fields, tabs, wizards and modal dialogs.

It is not easy to build such applications. Today, most of these apps are built with one of the popular Javascript frameworks like Angular or React on the client side, and with REST API on the server side. You need to deal with things like validation, localization, date & time formatting, error handling, API versioning and plenty of other features. And if you don't have any previous experience with JavaScript or TypeScript, you will need to learn a lot of new stuff – the language itself, many new libraries and tools like Require JS, webpack or npm.

In this article, I would like to show you an open source framework called DotVVM. It is an MVVM framework based on ASP.NET and its purpose is to make building these complicated websites easier. In most cases, you won’t even need to know Javascript. You will be able to stay in Visual Studio and use C#, HTML and CSS to do all the magic.

DotVVM supports both ASP.NET (thanks to OWIN) and ASP.NET Core and is actively developed on GitHub.

> I am one of the authors of DotVVM. I started this project more than 2 years ago. I have built a lot of web applications using Knockout, Angular, and React before, and I was not satisfied with the number of new things you need to learn and understand in order to build a simple “Hello world” application. That’s why I have decided to build this framework which abstracts the developer from the Javascript part.

How to Start

The first thing you need to start with DotVVM is to install a free Visual Studio extension called DotVVM for Visual Studio. You will need Visual Studio 2017 – the Community Edition is sufficient.

> There is also a commercial version of the extension which has more features (better IntelliSense for example). But for the purpose of this article, the free extension is good enough.

In order to create a new DotVVM website, you can use the File / New Project window. The extension adds several project templates to Visual Studio.

There is a template for .NET Framework and OWIN (the good old ASP.NET), and there are two templates for ASP.NET – one for .NET Core and one for full .NET Framework.

In this article, I will be using the .NET Core version of DotVVM which works on all platforms.

Sample Application

Instead of creating a new project from scratch, you can download and open the sample app from GitHub. I have already prepared a simple Entity Framework Core model, some sample data and a simple business layer with methods to provide everything the application needs.

> Please do not use this application as an example how a large line of business application should be architected. The purpose of this sample is to demonstrate some of the DotVVM features, like data-binding and validation, which may be useful in real line of business web applications.

The sample application assumes that you have SQL Server Express edition installed. If you don’t, look in the Database/AppDbContext.cs file and change the connection string which is hard-coded in the file.

MVVM

There are several client-side libraries which let you build web UI using the Model-View-ViewModel pattern. One of them is Knockout JS and DotVVM is something like a layer on top of Knockout JS.

View

DotVVM comes with its own HTML-based syntax for views. The views in DotVVM have the .dothtml file extension and the HTML syntax is extended with two main concepts – the server controls and the data-binding expressions.

<dot:TextBox Text="{value: PersonName}" />

The <dot:TextBox> element represents a DotVVM TextBox control. The value of the Text attribute is a data-binding expression which tells DotVVM to bind the control to the PersonName property in the viewmodel (which I will describe later).

When the server receives a HTTP request pointing to this page, DotVVM processes the .dothtml file and renders the following HTML code,

<input type="text" data-bind="value: PersonName" />

This is a syntax used by the Knockout JS library (which is added to the page). DotVVM also prepares the viewmodel object and passes it to the Knockout library so the data-binding expressions in the page can work.

ViewModel

In MVVM, the purpose of the viewmodel is to maintain the state of the view and respond to actions made by the user.

In DotVVM, you declare the viewmodel as a C# class with public properties and methods.

Every value which the user can change in the page should be stored in some public property in the viewmodel. For example, every form field should have its own property which maintains the value of the form field. Additionally, any state information the page needs must be exposed as a public property in the viewmodel.

All actions that the user can do in the page (clicking a button) are bound to public methods in the viewmodel.

<dot:Button Text="Save" Click="{command: SaveChanges()}" />

In DotVVM, every page must have its viewmodel. That’s why each DotVVM page starts with the @viewModel directive. This directive specifies the viewmodel type.

By default, the views are placed in the Views directory and the viewmodels are placed in the ViewModels directory. However, you don’t need to follow this convention.

> DotVVM needs to serialize the viewmodel in JSON to be able to include it in the HTML it renders. That’s why any state information needs to be a public property. Otherwise the JSON serializer would just ignore it.

Model

Model refers to the objects and methods provided by the “business layer”. In my sample application, you can find the following folders:

The Database folder contains the Entity Framework DbContext object and the entitity classes. This represents the data access layer.

The Services folder contains two classes which provide all the data for the user interface. You can find methods like GetProducts or GetOrderDetail

The Model folder contains object which are used in the views. The DTO suffix stands for the Data Transfer Object. The purpose of these objects is to represent the data. There are no methods in the DTO classes. The DTO objects are returned by the classes in the Services

To keep our code clean, you should not put the “business logic” of the application in the viewmodel. Instead, all the logic should be implemented in the service classes, and the viewmodel should only call methods from these “services”.

Rendering a GridView

DotVVM comes with several built-in controls which cover the basic stuff needed in the line of business applications. The GridView control can be used to render a table with data and it supports server-side paging and sorting.

You can see that the GridView control is bound to the Orders collection in the viewmodel. This collection contains all the rows displayed in the grid.

The control specifies two columns – the first displays the Id property of every order, the second displays the CreatedDate and uses the d format string (which stands for a default short date format). The syntax of format strings for dates and numbers is exactly the same as in .NET Framework.

Notice that I have added the class attribute to a DotVVM GridView control. This CSS class will be appended to the <table> element which is rendered by the control.

To render a clickable e-mail address in the table cells, I use the template column,

Notice that you can use data-bindings also with regular HTML elements (such as <a>) and that you can use simple C# expressions in the data-bindings, not just the property names.

In order to print a value of a property in an HTML element, you need to use double curly braces, because the single curly braces have special meaning e.g. in the script or style elements.

If the binding expression is a value of an HTML attribute, you can use just one.

Working with Data

In order to provide data to the GridView control, you need to declare a collection in the viewmodel and load the data to it.

You can use any .NET of the collections: array, List<OrderDTO> or any other IEnumerable. If you do so, you would need to handle the sorting and paging of the records by yourself. You will need to keep track of the page at which the user is and the track of the column which is used for sorting. This information is part of the page state and therefore there should be several properties in the viewmodel to represent this information.

To make this easier, DotVVM comes with the GridViewDataSet<T> class. It is a collection which stores its paging (page index, page size and the total number of records) and sorting (sort column and direction) parameters together with the current page of the records.

You can declare the Orders property in the viewmodel, initialize it with the new GridViewDataSet instance and set the default parameters for the paging and sorting.

public GridViewDataSet<OrderListDTO> Orders { get; set; }

= new GridViewDataSet<OrderListDTO>()

{

PagingOptions =

{

PageSize = 20

},

SortingOptions =

{

SortExpression = "CreatedDate",

SortDescending = true

}

};

The GridViewDataSet data can be loaded directly from an IQueryable object which is returned by the Entity Framework queries.

Notice that I don’t use the Order entity from the Entity Framework model, but I use the OrderListDTO object instead. This object contains only the properties I really need in the GridView.

> Remember that DotVVM serializes the viewmodel in JSON and sends it to the client. Be careful not to expose sensitive data like password hashes. It is very similar how you’d build a REST API.

Loading Data

You can override the PreRender method in the viewmodel and use it to load the data into the collection. The PreRender method is called by the framework right before the HTML is rendered and it is a good place to grab the data from the database.

You can see that I use the OrderService class to get the IQueryable<OrderListDTO> object. Then I pass it to the LoadFromQueryable method which applies the OrderBy, Skip and Take methods automatically, so I don’t need to care about paging and sorting in my code: it just works.

Finally, I have added the DataPager control and bound it to the GridViewDataSet object. It renders a list of pages and lets the user switch between them.

<dot:DataPager class="pagination" DataSet="{value: Orders}" />

Editing Data

I see the main advantage of MVVM when I build forms. No matter how the form is complicated and structured, the two-way data-binding in MVVM allows to pass a complex object in the UI, have its values modified by the user, and returned the object back so I can save it.

The OrderDetailDTO object contains the information about the order and it includes a collection of the order items as well. The user interface will let the user to modify the order data and to add or remove the items. All the changes will be synchronized with the viewmodel immediately thanks to the two-way data-binding.

The form with all the form fields is declared like this,

<form class="form" DataContext="{value: EditedOrder}">

...

</form>

You can see that we set the DataContext property on the <form> element to the EditedOrder. It is the property of the viewmodel which holds the OrderDetailDTO object.

The DataContext binding tells DotVVM to evaluate all data-bindings inside the <form> element against the EditedOrder property. Thanks to this, you don’t need to use {value: EditedOrder.ContactEmail}. You may just use {value: ContactEmail}.

The first form fields are quite straightforward. I use the Literal control to print a value with a specific FormatString, and the TextBox control to render a text input field.

<div class="form-group">

<label class="control-label">Created Date</label>

<div>

<p class="form-control-static">

<dot:Literal Text="{value: CreatedDate}" FormatString="d" />

</p>

</div>

</div>

<div class="form-group">

<label class="control-label">E-mail Address</label>

<div>

<dot:TextBox class="form-control" Text="{value: ContactEmail}" />

</div>

</div>

To let the user add or remove the order items, I can use the Repeater control. This control is bound to a collection of objects (OrderItems) and it renders the ItemTemplate for each object in the collection.

<dot:Repeater DataSource="{value: OrderItems}">

<ItemTemplate>

<div class="form-group">

<dot:ComboBox DataSource="{value: _root.Products}"

SelectedValue="{value: ProductId}"

DisplayMember="Name"

ValueMember="Id"

class="form-control" />

</div>

<div class="form-group">

<dot:TextBox Text="{value: Quantity}"

class="form-control"/>

</div>

<div class="form-group">

<dot:Button Text="Remove"class="btn btn-default"

Click="{command: _root.RemoveOrderItem(_this)}" />

</div>

</ItemTemplate>

</dot:Repeater>

You can see that the template contains several controls. Let’s take a look at them individually.

<dot:ComboBox DataSource="{value: _root.Products}"

SelectedValue="{value: ProductId}"

DisplayMember="Name"

ValueMember="Id" />

The ComboBox control allows us to select the product. It is bound to the _root.Products collection. Why is that?

Remember that all data-bindings inside the Repeater are evaluated against the current collection item and the Repeater is inside an element with the DataContext property.

If you are inside the Repeater and want to access the parent scope, you can use the _parent variable in the binding. If you need to get to the root viewmodel of the page, you can use the _root variable.

The Products collection is declared in the page viewmodel, next to the EditedOrder property. That’s why I used _root.Products as the DataSource.

The second property I used, SelectedValue, identifies the currently selected product in the ComboBox. It is bound to the ProductId property of the order item. What's important is that it lies outside of the Products collection.

DisplayMember tells the ComboBox, which property from each product will represent the text of the ComboBox item displayed to the user.

And ValueMember denotes the product's property which will be put into SelectedValue when the user selects it.

The second control in the row is the TextBox which holds the Quantity value. Optionally, you can specify the FormatString property.

<dot:TextBox Text="{value: Quantity}" />

The third control in the row is a Button which removes the particular order item.

<dot:Button Text="Remove"

Click="{command: _root.RemoveOrderItem(_this)}" />

The RemoveOrderItem method is declared in the root page viewmodel (remember that the Data Transfer Objects should not contain any methods). However, this method needs to know which order item is deleted. I am using _this to pass the current data context (which is the order item because I am still inside the Repeater) as an argument to the method.

The method just removes the item from the collection,

publicvoid RemoveOrderItem(OrderItemDetailDTO item)

{

EditedOrder.OrderItems.Remove(item);

}

Validation

DotVVM uses the Data Annotations attributes and the Model Validation framework to define and enforce the validation rules.

For example, you can mark the ContactEmail property in the OrderDetailDTO with the following attributes to make sure that the user will enter a proper e-mail address,

[Required]

[EmailAddress]

public string ContactEmail { get; set; }

In order to provide custom error messages, you can use the ErrorMessage parameter. Alternatively, you may use the ErrorMessageResourceType parameter to bind it to a value from a resource file,

[Required(ErrorMessage = "The E-mail Address field is required!")]

Validation Controls

DotVVM validates the entire viewmodel by default. If the user presses a button with a command and the viewmodel is not valid, the command doesn’t get executed.

You can use the Validation.Enabled property to disable the validation for a particular button. For example, if the user removes an order item, the entire form doesn’t have to be valid, so I can safely disable the validation on this button,

<dot:Button Text="Remove"

Click="{command: _root.RemoveOrderItem(_this)}"

Validation.Enabled="false" />

To display all error messages for a particular viewmodel object, you can use the ValidationSummary control. The Validation.Target property tells the control which object should be validated.

<div class="form-group text-danger">

<dot:ValidationSummary Validation.Target="{value: _this}" />

</div>

> Because of performance reasons, the control does not validate the child objects in the target unless you turn the IncludeErrorsFromChildren property on. If you don’t set the target, no error messages show up because the validated properties are in a child object of the viewmodel.

Often you want to highlight the form fields which contain incorrect values. You can set the default validation parameters on the validated control itself, or on any of its parent elements (for example, on the entire form).

When any element inside the <form> element is bound to a property which is not valid, it will get the has-error CSS class. To bind the element to a property tha tis being validated, you can use the Validator.Value property.

<div class="form-group">

<label class="control-label">E-mail Address</label>

<div Validator.Value="{value: ContactEmail}">

<dot:TextBox class="form-control" Text="{value: ContactEmail}" />

</div>

</div>

The SaveChanges method in the viewmodel saves the changes to the database. It just takes the EditedOrder object and passes it to the order service class which does all the database operations using the Entity Framework.

Routing

After the object is saved, I call Context.RedirectToRoute to return the user back to the list of orders.

The free extension for Visual Studio includes basic IntelliSense and DotVVM project templates. There is also a commercial version which has IntelliSense in data-binding expressions, real-time error checking and much more.